Aliphatic polyamide resins, such as polyamide 6.6, are very strong resins well suited for molding of various articles; however, when such resins are filled with mineral fillers, injection molded articles made from such resins are disadvantageously sensitive to moisture, do not exhibit a very glossy surface and have a low heat distortion temperature.
PCT Patent Application WO 91/15537 discloses a semiaromatic, semicrystalline copolyamide made from terephthalic acid, hexamethylene diamine and 2-methyl pentamethylene diamine and containing glass fillers. Those copolyamides have low moisture pickup and a heat distortion temperature ("HDT") under load of 1MPa of at least 240.degree. C. In order to achieve this high HDT, the addition of highly reinforcing fillers such as glass fibers and a high degree of nucleation are necessary. However, such filled compositions do not provide a smooth, glossy surface in molded articles, particularly injection molded articles. For certain applications such as automotive head lamp reflector housings, surface gloss is an important requirement. For those kind of applications glass-filled compositions are often unsuitable for surface appearance. In addition, the use of glass fillers in such applications results in an unnecessary expense because HDT does not need to be 240.degree. C. or higher.
In addition, semicrystalline copolyamide compositions tend to crystallize very slowly. An indication of the rate of crystallization is the exothermic cold crystallization peak temperature ("T.sub.cc ") as measured by Differential Scanning Calorimetry ("DSC"). For example, to injection mold a resin that has a T.sub.cc of 170.degree. C., it is necessary to use a mold temperature of greater than about 170.degree. C. to allow the molded part to crystallize completely. However, such a high mold temperature is not desirable. Such high temperature molding requires high energy expenditures to heat the mold and may require expensive equipment which is often difficult to control. It is known in the art to nucleate such a copolyamide composition in order to lower its T.sub.cc to below 150.degree. C., which is considered to be a maximum acceptable commercial mold temperature.
On the other hand, the lowest practical limit for a suitable mold temperature that provides good crystallization of the injection molded part is generally determined by the glass transition temperature ("T.sub.g ") of the polymer. For a polymer with a T.sub.g of 120.degree. C., for example, the mold temperature should not be below about 120.degree. C. to achieve complete crystallization and stress free injection molded parts. Because T.sub.g also reflects a resin's high temperature performance, it is desirable to provide a copolyamide composition with an acceptable combination of T.sub.g and T.sub.cc.
Japanese published patent application (Kokai) 61-162550 of K. Hikami et al., published 1986 July 23, discloses polyamides formed from 0.4-0.6 mole of aromatic diamines and/or aromatic carboxylic acids per mole of polyamide-monomer forming composition. If the polyamide is formed from both terephthalic acid and isophthalic acid, then the ratio of those acids must be in the range of 4:6 to 6:4. A number of diamines are disclosed, including 2-methyl pentamethylene diamine, but all of the examples illustrating the invention are of copolymers of terephthalic acid, isophthalic acid and hexamethylene diamine.
A polyamide composition is therefore needed which provides a sufficiently high T.sub.g, a low T.sub.cc, a high HDT, reduced moisture sensitivity, and improved surface appearance, without losing the advantageous properties of prior art compositions. Such compositions can be molded at low temperatures to form articles which have good high temperature performance and are relatively free of stress.